1. Field of the Invention
The present invention relates to a so-called vibration wave motor, in which a travelling vibration wave is generated in a vibrating member to cause friction drive of a movable member maintained in contact with said vibrating member, and in particular to a driving circuit therefor.
2. Related Background Art
FIG. 1A schematically illustrates a known vibration wave motor, wherein provided are electromechanical energy converting elements 1a, 1b such as electrostriction elements or magnetostriction elements, composed of foe example, lead zirconium titanate (PZT) which are adhered to a vibrating member 2, composed of an annular elastic plate. The vibrating member 2, together with the electrostriction elements 1a, 1b, is supported by an unrepresented stator. A movable member 3 constitutes, in this example, an annular rotor, maintained in contact with the other side of the vibrating member 2. The electrostriction elements are arranged along the periphery of the vibrating member 2 in such a manner that those 1a of a group are displaced by 1/4 of the wavelength .lambda. of the vibration wave, with respect to those 1b of the other group. The electrostriction elements 1a, 1a, 1a, . . . of a group are arranged with a pitch equal to 1/2 of said wavelength, in such a manner that the immediately neighboring elements have mutually opposite polarities. The arrangement is also the same for the elements of the other group.
In the vibration wave motor of the above-explained structure, an AC voltage V.sub.0 sin .omega.T is supplied to the electrostriction elements of a group while an AC voltage V.sub.0 cos .omega.T of those of the other group, whereby said electrostriction elements cause extending-shrinking vibrations in such a manner that the phases thereof are inverted between the neighboring elements and are displaced by 90.degree. between two groups. These vibrations are transmitted to the vibrating member 2 to induce a bending vibration therein according to the pitch of the electrostriction elements 1a, 1b in such a manner that the vibrating member is projected at every other element while it is retracted at the positions of every other remaining element. Since the elements 1a of a group are displaced by 1/4 of the wavelength with respect to those 1b of the other group, the bending vibration proceeds along the arrangement of the electrostriction elements. During the application of the AC voltages, vibrations are generated in succession and propagate along the periphery of the vibrating member 2 as progressive bending vibration waves, thus inducing a movement of the movable member 3.
FIG. 1B shows a state in which a vibration detecting element 1c is provided on the vibrating member 2.
The electrostriction elements 1a, 1b are provided, on the front and rear faces thereof, with unrepresented electrodes for applying the AC voltages.
FIG. 2 shows a driving circuit for such a vibration wave motor, wherein provided a power supply 10; an oscillator unit 11; a 90.degree. phase shifter unit 12; operational amplifiers OP1, OP2; resistors R1-R4; a capacitor C1; and electrostriction elements 1a, 1b shown in FIG. 1.
The above-explained driving circuit functions in the following manner. In the oscillator unit 11, the voltage V+ at the positive input terminal is the operational amplifier OP1 is obtained by dividing the output voltage V.sub.out of the operational amplifier OP1 with the resistors R2, R3: EQU V+={R3/(R2+R3)}.multidot.V.sub.out ( 1).
On the other hand, the voltage V- at the negative input terminal of the operational amplifier OP1 is obtained by dividing said output with the resistor R1 and the impedance z of the electrostriction element 1a: EQU V-={z/(z+R1)}.multidot.V.sub.out ( 2).
The impedance z of the electrostriction element varies according to the vibrating frequency f, as shown in FIG. 3, wherein fr is the resonance frequency and fa is the antiresonance frequency. By differentiating the equation (2) with f, there can be obtained: ##EQU1##
Consequently the voltage difference V.sub.in =(V+)-(V-) of the input terminal voltages varies in the following manner according to the frequency f:
______________________________________ f &lt;fr fr fr&lt; ______________________________________ dz/df -- + dV-/df -- + V- decrease z/(z + R1) increase V.sub.in increase ##STR1## decrease ______________________________________
As will be seen in this table, the input voltage difference V.sub.in reaches a maximum at the resonance frequency fr of the electrostriction element. Thus, the resistors R1-R3 are so determined as to satisfy the above-mentioned relation. The Q factor in this state is equal to A.sub.0 .multidot.Q.sub.0 wherein Q.sub.0 is the Q factor of the electrostriction element, and A.sub.0 is the amplification factor of the operational amplifier OP1.
In the 90.degree. phase shifter unit 12, an integrating circuit composed of the operational amplifier OP2, resistor R4 and capacitor C1 supplies the output AC voltage of the operational amplifier, with a delay in phase of 90.degree., to the electrostriction elements 1b.
In such circuit, the output voltage of the operational amplifier OP1 approaches the voltage of the power supply 10, eventually giving rise to a distortion in the driving sinusoidal waveform, due to fluctuation in the impedance z of the electrostriction elements 1a caused by changes in the internal or external temperature etc., though such behavior depends on the response of the operational amplifier OP1.
Such distortion in the waveform increases high-order components, such as second and third order components, of the resonance frequency. Such high-order components scarcely contribute to the drive but induces unnecessary vibration, thus decreasing the output power of the motor and reducing the efficiency thereof.
Also, there are drawbacks such as fluctuations in the revolution of the motor in a sudden change in the event the load or insufficient torque at the start-up.